U.S. patent application number 11/340546 was filed with the patent office on 2006-08-17 for organic el element, method for fabricating the same and organic el display device.
This patent application is currently assigned to OPTREX Corporation. Invention is credited to Koretomo Harada, Kazuhiro Monzen, Nobuhiro Nakamura, Shinju Otani.
Application Number | 20060182996 11/340546 |
Document ID | / |
Family ID | 36816014 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060182996 |
Kind Code |
A1 |
Nakamura; Nobuhiro ; et
al. |
August 17, 2006 |
Organic EL element, method for fabricating the same and organic EL
display device
Abstract
An organic EL element is provided which is capable of
restraining chrominance non-uniformity caused by a film thickness
distribution of an applied film, of having good display quality, of
reducing the driving voltage, and of having interlayer
short-circuit endurance. The organic EL element according to one
mode of the present invention comprises an anode 11, a cathode 12,
and an organic EL layer 13. The organic EL layer comprises a hole
injection layer 131 and a hole transport layer 132. The hole
injection layer includes organic-thin-film-forming-molecules and
dopants oxidizing the organic-thin-film-forming-molecules, the
dopants having a reduction potential of 0.5 to 0.85 V with respect
to a standard hydrogen electrode, and the hole transport layer
having an ionization potential of 8.5.times.10.sup.-19 J (5.3 eV)
or below.
Inventors: |
Nakamura; Nobuhiro;
(Yokohama-shi, JP) ; Monzen; Kazuhiro;
(Arakawa-ku, JP) ; Harada; Koretomo; (Arakawa-ku,
JP) ; Otani; Shinju; (Arakawa-ku, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
OPTREX Corporation
Arakawa-ku
JP
|
Family ID: |
36816014 |
Appl. No.: |
11/340546 |
Filed: |
January 27, 2006 |
Current U.S.
Class: |
428/690 ;
313/504; 313/506; 427/66; 428/212; 428/917 |
Current CPC
Class: |
Y10T 428/24942 20150115;
H01L 51/506 20130101; H01L 51/5048 20130101 |
Class at
Publication: |
428/690 ;
428/917; 428/212; 313/504; 313/506; 427/066 |
International
Class: |
H01L 51/50 20060101
H01L051/50; H01L 51/56 20060101 H01L051/56 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 27, 2005 |
JP |
2005-019015 |
Claims
1. An organic EL element comprising: an anode, a cathode, and an
organic EL layer disposed between the anode and the cathode; the
organic EL layer comprising a first organic thin film in contact
with the anode, and a second organic thin film in contact with the
first organic thin film; the first organic thin film including
organic-thin-film-forming-molecules and dopants oxidizing the
organic-thin-film-forming-molecules, the dopants having a reduction
potential of 0.5 to 0.85 V with respect to a standard hydrogen
electrode; and the second organic thin film having an ionization
potential of 8.5.times.10.sup.-19 J or below.
2. The organic EL element according to claim 1, wherein the
ionization potential of the organic-thin-film-forming-molecules of
the first organic thin film is lower than that of the second
organic thin film by 3.2.times.10.sup.-20 J or above.
3. The organic EL element according to claim 1, wherein the first
organic thin film has a career concentration of 5.times.10.sup.18
(1/cm.sup.3) or above.
4. The organic EL element according to claim 1, wherein the
organic-thin-film-forming-molecules of the first organic thin film
are water-insoluble.
5. The organic EL element according to claim 1, wherein the
organic-thin-film-forming-molecules of the first organic thin film
have a molecular weight of 1,000 or above.
6. The organic EL element according to claim 1, wherein the dopants
of the first organic thin film comprise organic acid.
7. The organic EL element according to claim 6, wherein the dopants
of the first organic thin film comprise a benzenesulfonic acid
derivative.
8. The organic EL element according to claim 1, wherein the dopants
in the first organic thin film have a molecular weight of 10,000 or
below.
9. The organic EL element according to claim 1, wherein the first
organic thin film comprises a thin film, which is disposed by
applying a liquid containing the
organic-thin-film-forming-molecules and the dopants.
10. An organic EL display device comprising a plurality of organic
EL elements defined in claim 1.
11. A method for fabricating an organic EL element, comprising
disposing an anode on a substrate, disposing an organic EL layer in
contact with the anode, and disposing a cathode in contact with the
organic EL layer; the step for disposing the organic EL layer
comprising: applying a liquid on the anode to dispose a first
organic thin film in contact with the anode, the liquid containing
organic-thin-film-forming-molecules and dopants oxidizing the
organic-thin-film-forming-molecules; and disposing a second organic
thin film in contact with the first organic thin film; the dopants
in the first organic thin film having a reduction potential of 0.5
to 0.85 V with respect to a standard hydrogen electrode; and the
second organic thin film having an ionization potential of
8.5.times.10.sup.-19 J or below.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Filed of the Invention
[0002] The present invention relates to an organic EL
(Electroluminescence) element, a method for fabricating the same
and an organic EL display device, in particular a composition of
the organic EL layer in an organic EL element.
[0003] 2. Discussion of Background
[0004] For recent years, research and development has been actively
conducted on organic EL display devices using an organic EL
element. Organic EL display devices are expected to be the next
generation of display devices because of having a wider viewing
angle range and faster response than liquid crystal display devices
and because of organic substances having a wide variety of light
emission properties. An organic EL element used in organic EL
display devices includes has anodes, cathodes disposed so as to
confront the anodes, and an organic EL layer disposed between the
anodes and the cathodes. Typically, the anodes, the organic EL
layer and the cathodes are laminated in this order from a substrate
surface.
[0005] The organic EL layer has a monolayered structure or a
multilayered structure. When the organic EL layer has a
multilayered structure, the organic EL layer includes organic thin
films, such as an organic light emitting layer, a hole injection
layer and a hole transport layer. The organic EL element is a
current-driven display element, which emits light by itself when a
current is supplied to the organic EL layer disposed between an
anode and a cathode. A position where an anode, the organic EL
layer and a cathode are overlapped with one another serves as a
display pixel.
[0006] When organic substances are laminated on electrodes disposed
on a substrate, organic materials are vacuum-deposited to form the
organic thin films in some cases. In a case of vapor-depositing the
organic materials, when an electrode as a underneath layer for the
organic thin films has a foreign material adhering thereto, or a
projection or a recess formed on a surface thereof, an organic thin
film fails to be formed in a desired state because of being
adversely affected by the presence of such a foreign material, a
projection or a recess in some cases.
[0007] As a method for solving this problem, there has been known a
wet application method (hereinbelow, referred to as the application
method). The application method is a technique wherein respective
organic materials for forming the respective organic thin films are
dispersed or dissolved in respective liquids, and the respective
organic materials are applied as the respective solutions to cover
such a foreign material, a projection, a recess or the like,
thereby to bring the organic thin films into a desired state. For
example, JP-A-2001-351779 discloses in paragraphs 0012 to 0017 that
at least one of organic thin films is formed by the application
method.
[0008] Examples of the application method are an offset printing
method, a relief printing method and a mask spray method. In the
offset printing method and the relief printing method, a thin film,
which comprises a solution containing an organic material dispersed
or dissolved in a solvent, is formed only in certain areas. In the
mask spray method, e.g., a glass mask or a metal mask, which has
apertural areas formed therein so as to conform to desired areas,
is positioned, and each solution with an organic material dispersed
or dissolved therein is sprayed. In the latter case, each of the
solutions is atomized by dispersing each of the solutions in a
gaseous medium, such as a nitrogen gas, or by using a two-fluid
nozzle or the like.
[0009] Various examples of the organic materials used in such
application methods include polyparaphenylenevinylene (PPV),
polythiophene and polypyrrole. On the other hand, there is a
technique wherein an oxidizing agent is doped to create holes in
order to improve the conductivity of a thin film formed by use of
such an organic material. Examples of the oxidizing agent include
Lewis acid, protonic acid, a transition metal compound, electrolyte
salt, and a halogen compound.
[0010] The formation and the properties of organic thin films
disposed by the application method are disclosed in, e.g., "Organic
EL material Technique" Chapter 5 under the editorship of Yoshiharu
SATOH, published by CMC press, May in 2004. According to this
non-patent document, adequate polymer organic materials and dopants
can be used to obtain not only an effect of restraining element
electrodes from being short-circuited due to surface smoothness
provided by the application method but also an effect of reducing
the driving voltage of the element.
[0011] However, when an organic EL element contains moisture, the
moisture diffuses in the organic EL element to form a non-emissive
area, or the moisture in the organic EL element promotes a
reduction in luminance to reduce display quality in some cases.
[0012] A dopant material used as the oxidizing agent needs to have
oxidizability enough to be capable of oxidizing an applied organic
material as well as having a tendency to have higher moisture
absorption as the oxidizability becomes higher. Accordingly, it is
preferred to use dopants having low moisture absorption, i.e.,
dopants having low oxidizability in order to avoid an adverse
effect caused by moisture in an organic EL element. Such dopants
having low moisture absorption may comprise an organic acid, such
as benzenesulfonic acid and toluenesulfonic acid.
[0013] However, the inventors have found that when dopants having
low moisture absorption as stated above is used, display
non-uniformity becomes prominent according to the thickness
distribution of an applied film. In this regard, detailed
explanation will be made. First, ITO was used to dispose anodes,
and a spray method was utilized to dispose a layer of PTPDEK
(represented by Chemical Formula 2), using TBPAH (represented by
chemical Formula 1) as dopants. On the layer of PTPDEK, a hole
transport layer was disposed, using PPD (represented by Chemical
Formula 3). ##STR1##
[0014] In this case, although display non-uniformity did not become
prominent according to the film thickness distribution of the layer
of PTPDEK, the luminance lifetime was short. One of the causes is
supposed to be that TPBAH has high moisture absorption. While the
ionization potential of TPBAH is as high as 9.6.times.10.sup.-9 J
(6 eV), the moisture absorption of TPBAH is high. It is supposed
that the excited state of Alq.sub.3 is quenched since the applied
film contains much residual moisture.
[0015] Next, a similar organic EL element was fabricated, changing
the dopants from the above-stated TBPAH to sulfosalicylic acid,
which has low moisture absorption, in order to reduce the amount of
residual moisture in the applied film. The degradation in the
luminance lifetime of the element was measured, and it was revealed
that the luminance lifetime was improved in comparison with the
element using TBPAH.
[0016] However, it was acknowledged that this element using
sulfosalicylic acid was subjected to chrominance non-uniformity
according to the thickness distribution of the applied film.
Additionally, it was revealed that the driving voltage of this
element increased in comparison with the above-stated element using
TPBAH as dopants. The chrominance non-uniformity was caused so that
a portion where the applied film was thick was dark while a portion
where the applied film was thin was light. From this point of view,
it is estimated that chrominance non-uniformity became visible
since the resistance of the film disposed by the application method
was highlighted for some reason.
[0017] As explained above, it is expected to increase interlayer
short-circuit endurance by the application method.
[0018] On the other hand, when dopants having high oxidizability
are used to reduce the driving voltage of an organic EL element, a
non-emissive area is formed, or the luminance lifetime is
deteriorated by an adverse effect caused by moisture contained in
the element. Conversely, when dopants having low oxidizability is
used, display non-uniformity becomes prominent according to the
thickness distribution, though the formation of a non-emissive area
or a degradation in the luminance lifetime is suppressed.
[0019] The present invention is proposed under the circumstances
stated above. It is an object to improve the interlayer
short-circuit endurance of an organic EL element, to restrain a
non-emissive area from being formed or luminance lifetime from
being deteriorated, to reduce the driving voltage and to suppress
display non-uniformity.
[0020] The inventors have been dedicated to making research and
development, and have found that the above-stated chrominance
non-uniformity can be avoided by a combination of an organic
material used for organic multi-layer thin films, dopants and an
organic material disposed on the organic multi-layer thin films.
The inventors have also found that it is possible to reduce the
driving voltage of an organic EL element and to obtain an organic
EL element having interlayer short-circuit endurance.
[0021] According to a first aspect of the present invention, there
is provided an organic EL element comprising an anode, a cathode,
and an organic EL layer disposed between the anode and the cathode;
the organic EL layer comprising a first organic thin film in
contact with the anode, and a second organic thin film in contact
with the first organic thin film; the first organic thin film
including organic-thin-film-forming-molecules and dopants oxidizing
the organic-thin-film-forming-molecules, the dopants having a
reduction potential of 0.5 to 0.85 V with respect to a standard
hydrogen electrode; and the second organic thin film having an
ionization potential of 8.5.times.10.sup.-19 J or below. Molecules
in the interface of the second organic thin film with the first
organic thin film can be oxidized even by low oxidizability dopants
to lower the energy barrier between the first organic thin film and
the second organic thin film since the use of such low
oxidizability dopants restrains an adverse effect from being caused
by moisture in the organic EL element and since the second organic
thin film comprises a material having a low ionization
potential.
[0022] According to a second aspect of the present invention, the
ionization potential of the organic-thin-film-forming-molecules of
the first organic thin film is lower than that of the second
organic thin film by 3.2.times.10.sup.-20 J or above in the organic
EL element recited in the first aspect. By this arrangement, it is
possible to significantly improve the injection ability of holes
from the anode.
[0023] According to a third aspect of the present invention, the
first organic thin film has a career concentration of
5.times.10.sup.18 (1/cm.sup.3) or above in the organic EL element
recited in the first or the second aspect. By having such a career
concentration, it is possible to sufficiently lower the energy
barrier between the first organic thin film and the second organic
thin film the energy barrier and to greatly offer effects of
suppressing chrominance non-uniformity and of reducing the driving
voltage.
[0024] According to a fourth aspect of the present invention, the
organic-thin-film-forming-molecules of the first organic thin film
are water-insoluble in the organic EL element recited in the first
to the third aspect. By this arrangement, it is possible to
suppress the amount of moisture contained in a thin film.
[0025] According to a fifth aspect of the present invention, the
organic-thin-film-forming-molecules of the first organic thin film
have a molecular weight of 1,000 or above in the organic EL element
recited in the first to the fourth aspect. By this arrangement, it
is possible to suppress non-uniformity in a film thickness and to
improve coatability with respect to unevenness of the anode.
[0026] According to a sixth aspect of the present invention, the
dopants of the first organic thin film comprise organic acid in the
organic EL element recited in any one of the first to the fifth
aspect. Organic acid is effective as low moisture absorption
dopants. According to a seventh aspect of the present invention,
the dopants of the first organic thin film comprise a
benzenesulfonic acid derivative in the organic EL element recited
in the sixth aspect. Such a sulfonic acid derivative is a preferred
material because of having both properties of oxidizability and low
moisture absorption.
[0027] According to an eighth aspect of the present invention, the
dopants in the first organic thin film have a molecular weight of
10,000 or below in the organic EL element recited in any one of the
first to the seventh aspect. By using such dopants, it is possible
to greatly expand the selection range of the solvent.
[0028] According to a ninth aspect of the present invention, the
first organic thin film comprises a thin film, which is disposed by
applying a liquid containing the
organic-thin-film-forming-molecules and the dopants in the organic
EL element recited any one of the first to the eighth aspect.
According to a tenth aspect of the present invention, there is
provided an organic EL display device comprising a plurality of
organic EL elements recited in any one of the first to the ninth
aspect.
[0029] According to an eleventh aspect of the present invention,
there is provided a method for fabricating an organic EL element,
comprising disposing an anode on a substrate, disposing an organic
EL layer in contact with the anode, and disposing a cathode in
contact with the organic EL layer; the step for disposing the
organic EL layer comprising applying a liquid on the anode to
dispose a first organic thin film in contact with the anode, the
liquid containing organic-thin-film-forming-molecules and dopants
oxidizing the organic-thin-film-forming-molecules; and disposing a
second organic thin film in contact with the first organic thin
film; the dopants in the first organic thin film having a reduction
potential of 0.5 to 0.85 V with respect to a standard hydrogen
electrode; and the second organic thin film having an ionization
potential of 8.5.times.10.sup.-19 J or below.
[0030] In accordance with the present invention, it is possible to
obtain an organic EL element, which combines high display quality,
a low driving voltage and interlayer short-circuit endurance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0032] FIG. 1 is a cross-sectional view of the organic EL element
according an embodiment of the present invention;
[0033] FIG. 2 is a schematic top plan view of an typical example of
a substrate with the organic EL display panel according to this
embodiment;
[0034] FIG. 3 is a cross-sectional view of a portion of the organic
EL display panel taken along line A-A of FIG. 2;
[0035] FIG. 4 is a flowchart showing a method for fabricating the
organic EL display panel according to this embodiment; and
[0036] FIG. 5 is a flowchart showing a method for fabricating the
organic EL layer according to this embodiment.
[0037] Now, an embodiment, to which the present invention is
applicable, will be described. The following description is
provided for explanation of the embodiment of the present
invention, and the present invention is not limited to the
embodiment described below.
[0038] FIG. 1 is a schematic cross-sectional view showing an
typical example of the structure of the organic EL
(Electroluminescence) element of this embodiment. The organic EL
element 1 has a laminated structure, which comprises an anode 11, a
cathode 12 disposed to confront the anode 11, and an organic EL
layer 13 disposed between the anode 11 and the cathode 12. The
anode 11 comprises a transparent conductive film made of ITO or the
like. The cathode 12 comprises a metal material, such as
aluminum.
[0039] The organic EL layer 13 has a multilayered structure, which
comprises a plurality of laminated thin films. In the typical
example shown in FIG. 1, the organic EL layer 13 has a four-layered
structure, which comprises a hole injection layer 131, a hole
transport layer 132, a light emitting layer 133 and an electron
injection layer 134 sequentially laminated in this order from a
side of the anode 11. The hole injection layer 131 is one example
of a first organic thin film in contact with the anode 11, and the
hole transport layer 132 is one example of a second organic film in
contact with the first organic thin film.
[0040] The hole injection layer 131 comprises an organic thin film,
which may be disposed on the anode 11 by an application method,
such as a spray method. The application method is a technique
wherein an organic material is dispersed or dissolved in a liquid,
and the organic material is applied to dispose a desired organic
thin film. The application method can be utilized to cover a
foreign material, a projection, a recess or the like on the anode
11, thereby to avoid the formation of interlayer short-circuit in
the organic EL element. Other application methods than the spray
method are also applicable.
[0041] The organic material that is usable in the application
method is broadly classified into a water-soluble (or
water-dispersible) one and a water-insoluble one that can dissolve
in an organic solvent. When the hole injection layer 131 comprises
a water-soluble organic material, the amount of moisture, which is
brought into the organic thin film, increases, easily causing
adverse effects, such as luminance degradation. For this reason, it
is preferred that the organic material for the hole injection layer
131 be water-insoluble. Thus, the amount of moisture contained in
the organic thin film can be suppressed, restraining the formation
of a non-emissive area or luminance degradation caused by moisture
in the organic EL element 1.
[0042] It is also preferred that the
organic-thin-film-forming-molecules of the hole injection layer 131
be polymers having a molecular weight of 1,000 or above. When the
hole injection layer 131 is disposed by the application method, it
is possible to use an organic material having a small molecular
weight. However, by using an organic material having the
above-stated molecular weight, the occurrence of non-uniformity in
the is film thickness can be minimized, having excellent
coatability with respect to unevenness of the anode 11 and more
effectively avoiding the generation of interlayer
short-circuit.
[0043] Although it is typical that the hole transport layer 132 and
its subsequent layers are disposed by vacuum deposition, these
layers may be disposed by the application method if properly
designed. The electron injection layer 134 may comprise, e.g., LiF.
An electron transport layer may be disposed between the light
emitting layer 133 and the electron injection layer 134, being
independent from the light emitting layer 133. There is no
particular limitation to the material of the light emitting layer
133. The light emitting layer may comprise, e.g.,
tris(8-quinolinolate)aluminum (Alq.sub.3) and coumarin 6 serving as
the fluorescent pigment of a guest compound.
[0044] Now, the hole injection layer 131 and the hole transport
layer 132 will be described in detail. The hole injection layer 131
lowers the injection barrier of holes from the anode 11 to reduce
the driving voltage. The hole injection layer 131 according to this
embodiment contains organic-thin-film-forming-molecules, and
dopants for oxidizing such molecules. The dopants oxidize some of
the organic-thin-film-forming-molecules to chemically create holes,
thereby improving the conductivity of the hole injection layer
131.
[0045] In this embodiment, the dopants in the hole injection layer
131 are controlled to have a reduction potential set at 0.5 to 0.85
V with respect to a standard hydrogen electrode. Additionally, The
hole transport layer 132, which transports holes to the light
emitting layer 133, is controlled to have an ionization potential
of 8.5.times.10.sup.-19 J (5.3 eV) or below. The dopants have
higher moisture absorption as their oxidizability becomes higher.
When the reduction potential, which is an index showing the
oxidizability of the dopants, is set at 0.85 V or below, the amount
of the moisture that is caused to remain in the hole injection
layer 131 by the moisture absorption of the dopants can be
decreased, effectively suppress the formation of a non-emissive
area and luminance degradation in the organic EL element 1.
[0046] Some of the dopants, which are contained in the hole
injection layer 131, exist in the interface with the hole transport
layer 132 in contact with the hole injection layer. When hole
transporting materials are oxidized by dopants, holes are created
in the interface of the hole transport layer 132 close to the hole
injection layer 131.
[0047] The holes thus created lower the energy barrier between the
hole injection layer 131 and the hole transport layer 132, with the
result that variations in element resistance caused by variations
in the film thickness of the hole injection layer 131 can be
reduced. Simultaneously, the carrier concentration in the hole
transport layer 132 can also increase to decrease the resistance of
the entire organic EL element to a lower level. However, when the
dopants have lower oxidizability, the dopants cannot oxidize hole
transporting materials in a sufficient way, with the result that
display non-uniformity, which is caused by variations in the
element resistance caused by variations in the film thickness of
the hole injection layer 131, can be generated.
[0048] As explained above, in the organic EL element 1 according to
this embodiment, the reduction potential of the dopants in the hole
injection layer 131 as the first organic thin film is controlled to
be set at 0.5 V or above, and the ionization potential of the hole
transport layer 132 is controlled to be set at 8.5.times.10.sup.-19
J (5.3 eV) or below. By this arrangement, molecules in the
interface of the hole transport layer 132 with the hole injection
layer 131 are oxidized even by dopants having low oxidization, with
the result that the energy barrier between the hole injection layer
131 and the hole transport layer 132 can be lowered. Thus, in
despite of using dopants having low oxidization, resistance
variations in the interface of the hole transport layer 132 can be
reduced, minimizing the generation of display non-uniformity, which
is caused if there are resistance variations in the interface of
the hole transport layer.
[0049] As the dopants have a lower reduction potential (lower
oxidizability), the moisture absorption of the hole injection layer
is lower. However, when the oxidizability of the dopants is too
low, organic molecules, which exist in the hole injection layer and
in the interface of the hole transport layer with the hole
injection layer, cannot be oxidized. From this point of view, the
reduction potential of the dopants is preferably 0.6 to 0.85 V with
respect to the standard hydrogen electrode, more preferably 0.6 to
0.75 V with respect to the standard hydrogen electrode.
[0050] The dopants in the hole injection layer 131 preferably
comprise organic acid having low moisture absorption in order to
suppress the adverse effect caused by moisture in the organic EL
element 1. As the organic acid, a sulfonic acid derivative is a
particularly preferred dopant material since this derivative is
excellent in terms of having balanced properties of oxidizability
and moisture absorption. As the dopants have a smaller molecular
weight, the selection range of the solvent can be wider. From this
point of view, the molecular weight of the dopants in the hole
injection layer is preferably 10,000 or below, more preferably
1,000 or below.
[0051] It is preferred that the ionization potential of the
organic-thin-film-forming-molecules in the hole injection layer 131
be lower than the ionization potential of the hole transport layer
132 by 3.2.times.10.sup.-20 J (0.2 eV) or is above. By lowering the
ionization potential in the hole injection layer 131 to such a
level, it is possible to significantly improve the injection
ability of holes from the anode 11. When the ionization potential
in the hole injection layer 131 is lowered, the energy barrier
between the hole transport layer 132 and the hole injection layer
is generally raised. However, in this embodiment, dopants contained
in the hole injection layer 131 oxidize molecules in the interface
of the hole transport layer 132 with the hole injection layer (see
the hatched portion in FIG. 1), causing the energy barrier to
lower. Accordingly, it is possible to improve the injection ability
of holes in the hole injection layer 131 and the hole transport
layer 132 in their entireties.
[0052] It is preferred that the carrier concentration in the hole
injection layer 131 be 5.times.10.sup.18 (1/cm.sup.3) or above.
When-the carrier concentration is set at 5.times.10.sup.18
(1/cm.sup.3) or above, the energy barrier between the hole
injection layer 131 and the hole transport layer 132 can be lowered
in a sufficient way, more effectively exhibiting the effects of
suppressing display non-uniformity and of reducing the driving
voltage.
[0053] As explained above, by disposing the hole injection layer
131 in contact with the anode 11 by use of the application method,
it is possible to improve the interlayer short-circuit endurance.
By causing the hole injection layer 131 to contain dopants having
lower oxidizability, it is possible not only to reduce the driving
voltage but also to restrain the display quality of the organic EL
element from being lowered by residual moisture. By selecting
organic-thin-film-forming-molecules having a low ionization
potential as the hole transporting material disposed on the hole
injection layer 131, it is possible to minimize the occurrence of
resistance variations in the interface of the hole transport layer
132 and to restrain display non-uniformity from being caused by the
resistance variations, in spite of using dopants having low
oxidizability. Although explanation has been made about a case
wherein the organic EL layer 13 has a four-layered structure, the
organic EL element according to the present invention is not
limited to have such a structure.
[0054] Now, an organic EL display panel using an organic EL element
1 according to the present invention will be described, referring
to FIGS. 2 and 3. FIG. 2 is a schematic top plan view showing the
structure of an element substrate with the organic EL element
disposed thereon in the organic EL display panel 100 according to
this embodiment. FIG. 3 is a partial cross-sectional view of the
organic EL display panel l 00 taken along line A-A of FIG. 2. As
shown in FIG. 2, the organic EL display panel l 00 according to
this embodiment includes anode wires corresponding to anodes 11
(hereinbelow, referred to as the anode wires 11), anode
supplemental wires 2, cathode wires corresponding to cathodes 12
(hereinbelow, referred to as the cathode wires 12), cathode
supplemental wires 4, an insulating film 6, openings 5 formed in
the insulating film, cathode separators 7, contact holes 8 and the
substrate 10. As shown in FIG. 3, the organic EL display panel 100
also includes an organic EL layer 13, a desiccant 22, a counter
substrate 20 and a seal for encapsulation 23.
[0055] The substrate 10 may comprise a non-alkali glass substrate
(e.g. a product commercially available under the product name
"AN100" manufactured by Asahi Glass Company, Limited) or an alkali
glass substrate (e.g. a product commercially available under the
product name "AS" manufactured by Asahi Glass Company, Limited).
Although there is no limitation to the thickness of the substrate
10, it is preferred to use a substrate having a thickness of, e.g.,
0.7 to 1.1 mm.
[0056] The substrate 10 has the plural anode wires 11 disposed
thereon so as to extend in parallel to one another as shown in FIG.
2. It is preferred that the anode wires 11 comprise, e.g. ITO. The
anode supplemental wires 2 are electrically connected to the anode
wires 11 at edge portions of the anode wires 11, respectively. The
anode supplemental wires are disposed so as to extend from
connection portion with the anode wires 11 toward an edge portion
of the substrate. In is other words, the anode supplemental wires
are disposed in the same number as the anode wires 11. The anode
supplemental wires are disposed so as to extend in parallel to one
another as in the anode wires 11.
[0057] Each of the anode supplemental wires 2 serves as a metal pad
for connection with an external wire, such as an FPC (Flexible
Printed Circuit board) or a TCP (Tape Career Package), through an
anisotropic conductive film (hereinbelow, referred as "ACF") on a
portion close to the edge portion of the substrate 10. By this
arrangement, current is supplied to the anode wires 11 through the
anode supplemental wires 2 from a driving circuit externally
disposed.
[0058] The substrate also has the plural cathode wires 12 disposed
thereon so as to extend in-parallel to one another and
perpendicular to the anode wires 11 as shown in FIG. 2. The cathode
wires 12 normally comprise Al or an Al alloy. The cathode wires may
comprise alkali metal, such as Li, Ag, Ca, Mg, Y, In or an alloy
containing at least one of them. The cathode wires may also
comprise a transparent conductive film. The cathode wires are set
so as to have a thickness of about 50 to about 300 nm.
[0059] The cathode supplemental wires 4 are electrically connected
to the cathode wires 12 through the contact holes 8 at edge
portions of the cathode wires 12, respectively. The cathode
supplemental wires are disposed so as to extend from the edge
portions of the cathode wires 12 toward an edge portion of the
substrate. In other words, the cathode supplemental wires are
disposed in the same number as the cathode wires 12. The cathode
supplemental wires are disposed so as to extend in parallel to one
another as in the cathode wires 12. Each of the cathode
supplemental wires 4 serves as a metal pad for connection with an
external wire, such as an FPC or a TCP, on a portion close to the
edge portion of the substrate 10 with the cathode supplemental
wires disposed thereon, as in the anode supplemental wires 2. The
contact hole may be formed so as to have dimensions of, e.g., 200
.mu.m.times.200 .mu.m.
[0060] The above-stated cathode supplemental wires 4 and the
above-stated anode supplemental wires 2 may comprise a metal film
having-a multilayered structure or a monolayered structure. For
example, both supplemental wires may have a multilayered structure
wherein a Mo/Nb layer, an Al layer and a Mo/Nb layer are laminated
in this order from a side of the substrate 10.
[0061] The insulating film 6 with the openings is disposed on the
anode wires 11, the anode supplemental wires 2 and the cathode
supplemental wires 4 so as to partly cover these wires (see FIG. 2
and FIG. 3). Each opening 5 for a pixel is located at a position
where an anode wire 11 and a cathode wire 12 intersect each other
as viewed in a plan view. Each opening 5 for a pixel corresponds to
a pixel area. For example, the insulating film 6 with the openings
may have a film thickness of 0.7 .mu.m, and each opening 5 for a
pixel may have dimensions of 300 .mu.m.times.300 .mu.m.
[0062] The organic EL layer 13 is disposed on the anode wires 11
and the insulating film 6 with the openings and is configured so as
to be sandwiched between the anode wires 11 and the cathode wires
12 as shown in FIG. 3. The organic EL layer 13 normally has a
thickness of about 100 to about 300 nm. The organic EL layer 13 is
disposed so as to meet the conditions explained in reference to
FIG. 1. For example, the organic EL layer 13 comprises the hole
injection layer 131, the hole transport layer 132, the light
emitting layer 133 and the electron injection layer 134 as shown in
FIG. 1. The hole injection layer 131 contains
organic-thin-film-forming-molecules and dopants. The reduction
potential of the dopants and the ionization potential meet the
conditions explained in reference to FIG. 1
[0063] The cathode separators 7 are disposed so as to extend
parallel to the cathode wires 12 as shown in FIG. 2. The cathode
separators 7 play a role to spatially separate the plural cathode
wires 12 from one another in order to prevent the cathode wires 12
from being connected together. It is preferred that the cathode
separators 7 have an inverted tapered shape in section. The
inverted tapered shape means that the cross-sectional shape of the
separators (the cross-sectional shape seen from a direction of B in
FIG. 2) has wider cross-sectional widths (in the direction of B in
FIG. 2) as the distance from the substrate 10 increases. By this
arrangement, it is possible to spatially separate the plural
cathode wires 12 in an easy way in the step for disposing the
cathode wires 12 stated later since the sidewalls and the rising
portions of the cathode separators 7 are located in the shade. The
cathode separators 7 may have dimensions of 3.4 .mu.m in
height.times.10 .mu.m in width, for example.
[0064] The above-stated substrate 10 is bonded to the counter
substrate 20 through the seal 23 to encapsulate a space with the
organic EL layer 13 and the like disposed therein. Encapsulation is
performed in order to prevent the organic EL layer 13 from being
deteriorated by moisture in the air. The counter substrate 20 may
comprise a glass substrate having a thickness of 0.7 to 1.1 mm, for
example. The counter substrate may comprise the same material as
the substrate 10. The counter substrate 20 has the desiccant 22
disposed thereon so as to have a gap with the organic EL layer 13,
the cathode wires 12 and the like In other words, the desiccant 22
is disposed so as to be apart from the organic EL element 1
including the anode wires 11, the organic EL layer 13 and the
cathode wires 12.
[0065] The desiccant 22 may comprise a viscous moisture-absorbing
material having a certain viscosity, for example. Or, the desiccant
may comprise an organic metal compound, which is highly reactive
with moisture and is formed in a film shape. The desiccant may also
comprise an inorganic desiccant. When the desiccant 22 comprises a
viscous moisture-absorbing material, the viscous moisture-absorbing
material may be prepared by mixing a certain amount of absorbent in
an inactive liquid comprising fluorinated oil. Or, the viscous
moisture-absorbing material may be prepared by mixing a certain
amount of absorbent in an inactive gel material, such as a
fluorinated gel.
[0066] The absorbent may comprise a material capable of physically
or chemically absorbing moisture, such as activated alumina,
molecular sieves, calcium oxide and barium oxide. The viscous
moisture-absorbing material is prepared so as to have such a creamy
or gel viscosity to prevent the absorbent from freely flowing, and
the viscous moisture-absorbing material thus prepared is applied
and disposed at a certain position.
[0067] A method for fabricating the organic EL display according to
the present invention will be described, referring to FIGS. 4 and
5. The method described below is a typical example in the case of
the organic EL display. It should be noted that other methods are
applicable as long as they do not depart from the spirit of the
invention. FIG. 4 is a flowchart showing a fabrication process for
the organic EL display according to this embodiment.
[0068] In FIG. 4, a material for the anode wires is deposited as a
film on the substrate 10 in Step S1. The material for the anode
wires, which comprises, e.g., ITO, is uniformly deposited as a film
on the entire surface of the substrate by, e.g., sputtering or
vapor deposition.
[0069] Next, the deposited material for the anode wires is
patterned to form the anode wires 11 by a photolithographic step
and an etching step in Step S 2. The etching step may be performed
by either a wet-etching method or a dry etching method. For
example, the etching step is performed, using a phenol novolak
resin as a resist, by the wet-etching method. A solution with
hydrochloric acid and nitric acid mixed therein is used as a
processing liquid, and a solution with monoethanolamine and
dimethylsulfoxide mixed therein is used as a stripping liquid.
[0070] Next, a material for the supplemental wires is deposited as
a film on the anode wires by sputtering or vapor deposition in Step
S3. The material for the supplemental wires may comprise, e.g., a
metal material having a low resistance, such as Al or an Al alloy.
From the viewpoint of, e.g., improving adhesion with an underneath
layer and of preventing corrosion, the supplemental wires may
formed in a multilayered structure by disposing a barrier layer
made of TiN, Cr, Mo or the like as a upper or lower layer made of
an Al film. For example, the supplemental wire may be formed in a
multilayered structure of Mo/Al/Mo having a total thickness of 450
nm by a DC sputtering method.
[0071] Next, the material for the supplemental wires deposited in
the above-stated Step S3 is patterned to form the anode
supplemental wires 2 and the cathode supplemental wires 4 by a
photolithographic step and an etching step in Step S4. For example,
wet-etching is performed, using an etching solution with phosphoric
acid, acetic acid and nitric acid mixed therein. The materials for
the supplemental wires and the material for the cathode wires may
be sequentially patterned after the material for the anodes and the
material for the supplemental wires have been sequentially
deposited as films.
[0072] After that, a material for the insulating film, such as
photosensitive polyimide, is deposited as a film by, e.g.,
spin-coating in Step S5.
[0073] Next, the insulating film is patterned in Step S6.
Patterning is carried out so that the openings 5 for the respective
pixels serving an active area and the contact holes 8 are formed in
the insulating film. When photosensitive polyimide is used, the
insulating film 6 is patterned so as to have the openings 5 and the
contact holes 8 formed therein as shown in FIG. 2 and FIG. 3 by
performing a curing step after having performed an exposure step
and a development step.
[0074] Next, a material for the cathode separators is deposited as
a film in Step S7. For example, a photosensitive novolac resin, a
photosensitive acrylic resin or the like is deposited as a film by
spin-coating.
[0075] After that, the material for the cathode separators is
patterned in Step S8. By patterning, each of the cathode separators
7 is disposed in a gap to be located between adjacent cathode wires
17 so as to extend parallel with the cathode wires 12 as shown in
FIG. 2. It is preferred that the cathode separators 7 have an
inverted tapered shape in section (the cross-sectional shape seen
from the B direction in FIG. 2). When a negative photosensitive
resin is used, it is easy to form such an inverted tapered
structure in the exposure step since the cathode separators 7 have
a lower portion subjected to more insufficient photoreaction.
[0076] It should be noted that in order to provide surface
modification to portions of the ITO film exposed from the openings
5 formed in the insulating film, a step for irradiating oxygen
plasma or ultraviolet may be inserted before Step S 9 stated
later.
[0077] Subsequently, the organic EL layer 13 is disposed in Step
S9. Referring now to FIG. 5, the hole injection layer 131 is
disposed as the lowest layer by use of an application method in
Step S91. For example, the hole injection layer 131 is disposed by
a spray method. The hole injection layer 131 may be disposed by
using a solution with PTPDEK (5 mg/ml) and para-toluenesulfonic
acid (20 wt %) dissolved in cyclohexanone, for example. Next, the
solution is condensed and dried to be cured, disposing the hole
injection layer 131.
[0078] Subsequently, the other organic layers forming the organic
EL layer 13 are disposed as upper layers of the hole injection
layer 131 in Step S92. For example, the hole transport layer 132 is
disposed so as to have a film thickness of 50 nm by 2-TNATA
(represented by Chemical Formula 4). Additionally,
Alq(tris(8-hydroxyquinolinato)aluminum as the host compound of the
light emitting layer 133 and coumarin 6 as the fluorescent pigment
of the guest compound are simultaneously formed by vapor deposition
to dispose the light emitting layer 133 (also serving as the
electron transport layer) having a film thickness of 60 nm in Step
93. Subsequently, the electron injection layer 134 is disposed so
as to have a film thickness of 0.5 nm by forming e.g., Lif as an
upper layer of the light emitting layer 133 by vapor deposition in
Step S94. ##STR2##
[0079] Referring back to FIG. 4, a material for the cathode wires,
which is used for disposing the cathode wires, is accumulated by
e.g., mask vapor deposition in Step S10.
[0080] Next, a step for preparing the counter substrate, which
encapsulates the organic EL element 1, will be is described.
[0081] First, a desiccant housing portion is formed so as to have a
concave shape on the counter substrate 20 by, e.g., etching or
sandblast in Step S11.
[0082] Subsequently, the seal for encapsulation 23, which are made
of, e.g., a photo cationic polymerizable epoxy resin, is applied on
the surface of the counter substrate with the concave housing
portion in Step S 12. The cathode supplemental wires 4 and the
anode supplemental wires 2 are disposed so as to extend to outside
the seal for encapsulation 23 in order to be connected to a driving
circuit externally provided as stated later. After that, the
desiccant 22 is disposed in Step S13.
[0083] After that, the substrate 10 and the counter substrate 20
are bonded together in Step S14. Specifically, the substrate 10 and
the counter substrate 20 are aligned with each other, followed by
applying a pressure to both substrates and irradiating the
respective seals with UV light. Thus, the substrate 10 with the
organic EL element disposed thereon, and the counter substrate 20
are bonded together. As a result, the organic EL element 1 is
encapsulated.
[0084] Finally, the driving circuit and the like are mounted in
Step S 51. Edge portions of the cathode supplemental wires 4 and
the anode supplemental wires 2, which extend to outside the seal
for encapsulation 23, are bonded to the ACF and are connected to
the TCP with the driving circuit disposed thereon. Then, the
organic EL display panel 100 is mounted to a casing, completing the
fabrication of an organic EL display.
[0085] Now, the embodiment will be more specifically described,
referring to examples. The examples are not intended to narrowly
construe the present invention.
EXAMPLE 1
[0086] Example 1 will be explained, referring to Table 1. The
organic EL element was fabricated so as to have a laminated
structure, which comprised anodes, a first organic thin film (hole
injection layer), a second organic thin film (hole transport
layer), a third organic thin film (light emitting layer), a fourth
thin film (electron injection layer) and cathodes. The anodes were
made of an ITO film having 150 nm. The first organic thin film
(hole injection layer) was disposed, using a solution containing
PTPDEK (5 mg/ml) and para-toluenesulfonic acid (20 wt*) as the
dopants dissolved therein, by a spray method. Cyclohexanone was
used as the solvent. The second organic thin film (hole transport
layer) was made from 2-TNATA having a film thickness of 50 nm. The
third organic thin film (light emitting layer), and the fourth thin
film (electron injection layer) were made from Alq.sub.3 having a
film thickness of 60 nm and from Lif having a film thickness of 0.5
nm, respectively. The cathodes were made of an Al film having 80
nm.
[0087] The reduction potential of para-toluenesulfonic acid of the
first organic thin film is 0.75 V with respect to the standard
hydrogen electrode, and the ionization potential of 2-TNATA of the
second organic thin film is 8.2.times.10.sup.-19 J (5.1 eV) . And,
the ionization potential of PTPDEK as the
organic-thin-film-forming-molecules is 8.6.times.10.sup.-19 J (5.4
eV).
[0088] In the case of the element structure stated above, any
non-emissive area deriving from chrominance non-uniformity or
moisture, which reflected the film thickness distribution of an
applied film, was not recognized. The mobility found by mobility
measurement (TOF method) was 10.sup.-7 cm.sup.2/Vs.
[0089] Based on the current-voltage characteristics of the element
having a structure of ITO/first-organic-thin-film (10 nm)/Al and on
the mobility found as stated above, it was estimated that the
career concentration was about 5.times.10.sup.18 (1/cm.sup.3).
Based on this value, it is estimated that 2-TNATA was oxidized in
the interface between PTPDEK of 2-TNATA and the dopant layer of
para-toluenesulfonic acid, thereby suppressing chrominance
non-uniformity in this element.
[0090] In addition, with regard to the driving voltage, the voltage
capable of obtaining current of 500 mA/cm.sup.2 decreased by about
2 V, in comparison with a case of using PPD having an ionization
potential of 8.6.times.10.sup.-19 J (5.4 eV) as the hole
transporting material. TABLE-US-00001 TABLE 1 first organic thin
film forming-molecules PTPDE molecular weight (15,000 to 25,000)
dopants para-toluenesulfonic acid reduction potential of 0.75 V
dopants ionization potential of 8.6 .times. 10.sup.-19 J (5.4 eV)
forming-molecules career concentration 5 .times. 10.sup.18
(l/cm.sup.3) second organic thin film forming-molecules 2-TNATA
ionization potential 8.2 .times. 10.sup.-19 J
EXAMPLE 2
[0091] In Example 2, the organic EL element was fabricated so as to
have a laminated structure, which comprised anodes, a first organic
thin film (hole injection layer), a second organic thin film (hole
transport layer), a third organic thin film (light emitting layer),
a fourth thin film (electron injection layer) and cathodes. The
anodes were made of an ITO film having 150 nm. When the first
organic thin film (hole injection layer) is disposed, 150 wt % of
sulfosalicylic acid as the dopants was first added to oligoaniline
units represented by Chemical Formula A and tetracarboxylic
dianhydride, the mixture was dissolved in a solution of
cyclohexanone, and the solution was applied as an applied film by a
spray method. After that, the applied film was baked at 250.degree.
C. for one hour to obtain organic-thin-film-forming molecules
represented by Chemical Formula B. ##STR3##
[0092] The second organic thin film (hole transport layer) was made
from HI406 (manufactured by Idemitsu Kosan Co., Ltd.), having a
film thickness of 50 nm. The third organic thin film (light
emitting layer) and the fourth thin film (electron injection layer)
were made from Alq.sub.3 having a film thickness of 60 nm and from
Lif having a film thickness of 0.5 nm, respectively. The cathodes
were made of an Al film having 80 nm.
[0093] As shown in Table 2, the reduction potential of
sulfosalicylic acid of the first organic thin film is 0.75 V with
respect to the standard hydrogen electrode, and the ionization
potential of HI406 of the second organic thin film is
8.4.times.10.sup.-19 J (5.2 eV). And, the ionization potential of
the organic-thin-film-forming-molecules represented by Chemical
Formula 2 is 8.2.times.10.sup.-9 J (5.1 eV).
[0094] In the case of the element structure stated above, any
non-emissive area deriving from chrominance non-uniformity or
moisture, which reflected the film thickness distribution of an
applied film, was not recognized. The mobility found by mobility
measurement (TOP method) was 10.sup.-7 cm.sup.2/Vs.
[0095] Based on the current-voltage characteristics of the element
having a structure of ITO/first-organic-thin-film (10 nm)/Al and on
the mobility found as stated above, it was estimated that the
career concentration was about 2.times.10.sup.19 (1/cm.sup.3).
Based on this value, it is estimated that HI406 was oxidized in the
interface of H 1406 with the first organic thin film, thereby
suppressing chrominance non-uniformity in this element.
[0096] In addition, with regard to the driving voltage, the voltage
capable of obtaining current of 500 mA/cm.sup.2 decreased by about
3 V, in comparison with a case of using PPD having an ionization
potential of 8.6.times.10.sup.-19 J (5.4 eV) as the hole
transporting material. The reason is supposed to be that the
injection ability of holes to the light emitting layer has been
improved since the ionization potential of HI406 as the hole
transporting material was larger than 2-TNATA by
1.6.times.10.sup.-20 J (0.1 eV). TABLE-US-00002 TABLE 2 first
organic thin film forming molecules Chemical Formula B molecular
weight (about 1,050) dopants sulfosalicylic acid reduction
potential of 0.75 V dopants ionization potential of 8.2 .times.
10.sup.-19 J forming-molecules career concentration 2 .times.
10.sup.19 (l/cm.sup.3) second organic thin film forming-molecules
HI406 ionization potential 8.4 .times. 10.sup.-19 J (5.2 eV)
[0097] The entire disclosure of Japanese Patent Application No.
2005-019015 filed on Jan. 27, 2005 including specification, claims,
drawings and summary is incorporated herein by reference in its
entirety.
* * * * *